1. Field of the Invention
The present invention relates to a bus competitive control apparatus which prevents delays of data due load resistances or load capacities in performing collision detection in a bus competitive control mode (system).
2. Description of the Prior Art
A description will be made with reference to FIGS. 1, 2 and 10 in terms of a conventional bus competitive control apparatus. FIG. 1 shows one example of data communication formats predetermined between common data circuits (lines). In FIG. 1, the format is composed of a transmission start signal area 1 for transferring the transmission start to all units coupled to the common data circuits, a priority determining area 2 for determining the priority at the time of collision in the bus competitive control mode, a communication control area 3, a communication data area 4, a communication data termination area 5 and a transmission completion area 6. Further, as illustrated in FIG. 2, a communication system is constructed such that units 12a to 12d having communication control functions communicable in the aforementioned common data communication format are coupled through transmission lines 10a to 10d and reception lines 11a to 11d to a common data circuit 9. As shown in FIG. 10, each of the units 12a to 12d comprises a control section 13 and a communication control section 14, and the communication control section 14 includes a transmission section 15 for transmission control to the common data circuit 9, a reception section 16 for reception control from the common data circuit 9, and a collision detection section 17 for priority determination (collision detection) at the time of collision in the bus competitive control mode.
Secondly, a collision detection operation of the aforementioned communication system in the bus competitive control mode will be described with reference to a data transmission flow chart of FIG. 9 in addition to FIGS. 1, 2 and 10. The transmission and reception of data are effected in the predetermined common data communication format as illustrated in FIG. 1. In the communication system in which the respective units 12a to 12d are coupled through the transmission lines 10a to 10d and the reception lines 11a to 11d to the common data circuit 9 as illustrated in FIG. 2, let it be assumed that the A and B units 12a and 12b generate transmission requests. The collision detection operation in the bus competitive control mode in this case will be described hereinbelow with reference to the data transmission flow chart of FIG. 9. In FIG. 9, prior to the communication, the A unit 12a and B unit 12b generating the transmission requests (step 19) respectively performs transmission frame assembling works (step 19) in accordance with the data communication format as shown in FIG. 1. Then, the A unit 12a and B unit 12b observe the transmission and reception status on the above-mentioned common data circuit and detect, on the basis of a communication termination signal from the communication data termination area 6 on the FIG. 1 data communication format, that the transmission and reception of data are completed between the other units, thereafter advancing from the transmission frame deferring state decision (step 20) to the transmission start (step 21) state. With this operation, the transmission start phases of the A and B units 12a and 12b are combined, and a transmission start signal area 1 shown in FIG. 6 is transmitted to the common data circuit 9, then proceeding to a priority determining area 2 to perform the collision detection (step 22) in FIG. 9. Here, let it be assumed that on this communication system the A unit 12a has been set to be higher in priority than the B unit 12b. First, a description will be made in terms of the operation of the A unit 12a having the higher priority. The A unit 12a, effecting the transmission start (step 21 ) in FIG. 9, enters into the priority determining area 2 to perform the collision detection (step 22) in FIG. 9 and confirms that a signal having a priority higher than the priority signal itself is not generated from the other units, then transmitting the communication control area 3, communication data area 4, communication data termination area 5 and transmission completion signal area 6 in FIG. 6 to the common data circuit 9 in FIG. 2. In FIG. 9, at the time of the completion of the transmission, the operation advances from the transmission completion decision (step 26) to a transmission error detection (step 27). Here, the operation goes to an abnormal termination (step 29) in the case of detection of an error and goes to a normal termination (step 28) in the case of no detection of the error, thereby terminating the transmission.
Further, a description will be made in terms of the operation of the B unit 12b having a priority lower than that of the A unit 12a. In FIG. 9, the B unit 12b, performing the transmission start (step 21 ), enters into the priority determining area 2 to effect the collision detection (step 22) so as to perform the collision detection. The B unit, detecting that a signal having a priority higher than its own priority signal is generated from the other unit, performs a transmission stop process (step 23) and then increases the trial count by 1 (step 24) to check whether it exceeds the trial count limitation (step 25). In the case of not exceeding the trial count limitation, the operation again advances to the transmission deferring state decision (step 20). On the other hand, if exceeding the trial count limitation, the operation proceeds to the abnormal termination (step 29).
When each of the units shown in FIG. 2 performs the above-described operation, in the above-mentioned communication system, the collision detection in the bus competitive mode can be realized, and a series of communication data transmissions from the transmission start signal area 1 to the transmission completion signal area 6 in FIG. 6 are effected with respect to the common data circuit 9 at a fixed transmission speed to perform the data transmission and reception between the respective units.
Since the conventional bus competitive control apparatus is arranged as described above, the collision detection in the bus competitive control mode is affected by delays of the transmitted communication data due to load resistances or load capacities of the common data circuit and hence the transmission is required to be effected at a speed including a portion corresponding to the delay whereby there is a problem that difficulty is encountered to increase the transfer speed for the transmission and reception between the units.